Enhanced 911 or E911 services are a North American Telephone Network feature used with 911 emergency calls. The goal is to provide a physical location associated with a caller making a 911 (emergency) call to a public safety answering point (PSAP). For 911 calls made on landlines, providing a location to the PSAP is straightforward. The carrier on which the call is made provides the calling number and its associated address to the PSAP. The address information is shown on the PSAP operator's screen during the call. The operator's screen often includes a visual map of the address as well, so the human operator can assist emergency vehicles in getting to the location. Integrating address information into PSAP's operations (databases) has been occurring since the 1970s.
With the advent of wireless communications, providing location information became a significant technical problem. Location information about wireless devices can be derived with various degrees of reliability and specificity. The first solutions derived location information using the location of the cell through which the wireless device or mobile station (MS) was communicating. Additional location information was derived from the MS's signals, which could include the signal's angle of arrival and the signal's time difference of arrival at different cells. Location determination solutions that use location information obtained from the network are called network-based solutions.
Many MSs are now equipped to receive location information directly. One common implementation uses GPS receivers in the MS. However, due to the memory and/or compute-intensive nature of deriving coordinates from the location information, the calculation to determine a set of coordinates from the location information may not be done on the MS, instead the MS sends its location information to the network. The network has the resources to derive coordinates, such as a latitude and longitude, using the location information sent by the MS. The network can also make use of network derived location information in addition to the location information provided by the MS when determining the MS's location coordinates. Location determination methods which use location information provided by the MS are called MS-based, or mobile-based, solutions. These solutions include methods that make use of resources and additional information from the network. MS-based location technologies that use GPS receivers in the MS and use the network for assistance data are usually implemented using assisted GPS, or AGPS. Although MSs using AGPS typically do not make their own position calculations, AGPS itself does not require that limitation.
Another MS-based location technology is Advanced Forward Link Trilateration (AFLT). To determine an MS's location, the MS takes measurements of signals from nearby cells (base stations) and reports the cells' pilot strength and pilot phases back to the network. This information is used by the network to triangulate a location of the MS. At least three surrounding base stations are needed to get optimal location information.
Hybrid location technologies may use both AGPS and AFLT. Hybrid location technologies are useful in urban areas where the GPS signal may be hard to detect or where there is considerable shading and interference.
Currently wireless networks include a network-based position determining entity (PDE) to calculate the mobile's position. The location information it uses may be from the network, the MS, or both; the PDE is used in both MS-based and network-based solutions.
When a 911 call is made from a mobile wireless device, the location information is provided by the wireless carrier to the PSAP from the PDE in the form of a mathematical representation of coordinates. However, there are numerous PSAPs around the country that do not support position information from the wireless carrier. If a connection is made to a PSAP that does not support coordinate-based location information, the network does not request location information from the MS and does not generate location coordinate information. Additionally, the MS will typically not generate location information unless it is requested by the network. Although progress is being made, there may always be PSAPs that can not support position information from wireless carriers.
There is a need for a wireless mobile device that allows an emergency caller to provide location information to a PSAP operator, when the responding PSAP system does not support the location information delivered by a wireless carrier.